I'm Totally agree with those 3, but do not forget the Tridente in Social Networking such as :
FaceBook, Twitter and G+, they are the most visited site on Blue Planet instead of Search Engines of course, therefore the search engines counts them fearfully and "befriend" sites that have most recognition in Social Networking.

"He who rule the social, rule the marketing land "

Google sees it as a deadly point therefore they had G+ to avoid being killed by Facebook in the Future.

That is my thought mate, what are yours ?

kind regards

xeniux